A thermal cycle of a heat engine that employs a quantity of gas as an operating medium can be described by reference to a pressure-volume (P-V) diagram. FIGS. 1 and 2 show P-V diagrams for two well-known thermal cycles, the Carnot cycle (FIG. 1), and the ideal Sterling cycle (FIG. 2).
The net energy delivered from one thermal cycle is the area of the loop swept out by the operating path in the P-V plane. In the course of each cycle, energy is delivered by the engine for part of the cycle, and is absorbed by the engine for the remainder of the cycle. For some parts of some cycles, energy is neither stored nor delivered. For instance, in the ideal Sterling cycle, mechanical energy is neither absorbed nor delivered during those parts of the cycle where the trajectory is parallel to the P-axis.
By necessity, part of the system used for extracting a net positive average power output must include a device for storing and returning energy out of and into the heat engine, on a cyclic basis. In conventional heat engines, this cyclic energy storage is accomplished by mechanical means, for example via the rotational inertia of a crankshaft with flywheel attached.